Drying kilns are used at saw mills and treatment plants to lower the moisture content of lumber to a selected desired level. The lumber fed into drying kilns is typically "green" lumber, cut from previously living trees and that typically has high moisture content. The lumber is dried usually by application of heat and airflow directed through and around layers of lumber arranged in stacks forming a charge within the kiln chamber.
Drying kilns are a faster alternative and more consistently controllable than air drying. Stacks of lumber may be effectively dried in just over two days, depending of course on the species of wood, initial moisture content, and other considerations that can affect drying time. On the other hand, air drying of the same material may take several months or more. Expediency is clearly an advantage in kiln drying over air drying, but need remains for efficient kiln systems that will provide for expeditious drying at low cost.
A "charge" of lumber is comprised of at least one stack of boards with layers separated by thin transverse slats termed "stickers." The stickers are used at spaced locations along the layers to form air passages in the stack. It is an objective in drying kilns to move drying air through the charge in an even manner so the individual boards will dry uniformly. Thus it is desirable to have substantially equal airflow through each of the spaces formed by the stickers. This is not possible at the top of the typical charge, since the top layer of lumber is exposed. As a result, various baffle arrangements have been devised to engage the top of the charge and prevent or control airflow across the top board layer.
One common baffle arrangement involves long baffle plates that are hinged to the ceiling or fan deck of the kiln and that can be pivoted up to positions clear of an incoming charge of lumber. Once the charge is positioned in the kiln, the baffles are pivoted down to engage the top longitudinal corners of the charge and effectively limit airflow across the top layer. In actual practice, the baffles function quite well but are difficult and time consuming to use. A kiln operator must go into the kiln, lift and lock the baffles (which are quite heavy) in an up position before the charge is moved in. Then, once the charge is in place within the kiln, the operator must lower the baffles to engage the top layer. Finally, when the drying process is complete, the operator must again go into the kiln, and raise the baffles to allow the charge of dried lumber to be removed. Shifting the baffles up and down in the kiln is not only hard physical labor (since the baffles are heavy), but the worker is exposed to dark, steamy, unsafe conditions in the kiln. It is not at all unusual for a baffle to accidently strip a board from the top of a charge and onto a worker below. It is also a common occurrence for someone to forget to lift the baffles before a dried charge is taken from the kiln. The baffles will then drag along the top layer of lumber and either be damaged by the lumber, push one or more boards off the top of the charge, or simply swing down to positions in a collision path with the next incoming charge of lumber.
As a solution to the baffle problem discussed above, attempts have been made to eliminate or modify the baffle construction in drying kilns. For example, U. S. Pat. No. 5,488,785 granted to George Culp on Feb. 6, 1996, recognizes the need to effectively control air passage over a charge of lumber in a dry kiln. The solution offered is provision of flat panels that are positioned on the top layers of the charge. Placement of the panels is a time consuming task, requiring the use of a fork lift truck to hoist and place the panels in place, then to remove the panels after drying.
Another attempted solution to the dry kiln baffle problems discussed above is disclosed in U.S. Pat. No. 4,972,604 granted to Leon Breckenridge on Nov. 27, 1990. This patent discloses the use of inflatable bladders that are positioned in a dry kiln and inflate to engage the charge of material to be dried, thereby confining the passage of air through the kiln chamber to prescribed passages. However, the time required to inflate the bags becomes a negative factor, as well as the propensity for the inflated bags to burst when inflated against sharp splinters. Still further, the high pressure pumps required to properly operate the bags adversely affects the overall cost of the baffling unit.
Still other attempts have been considered, including flexible cloth or canvass aprons draped loosely against the top surface of a charge within a kiln. Such drapes often snag, wear quickly, or may be too flexible to adequately control airflow in the drying chamber.
A need has therefore remained for a drying kiln baffle system that will operate to influence airflow within a drying kiln without requiring manual or automatic positioning. A need further remains for a baffle that is relatively safe to operate and that in normal use, will not drag lumber from a dry kiln charge as the charge is being moved into or out from the drying kiln.